It has previously been proposed to make crucibles of quartz glass by introducing a finely ground mass of crystalline quartz or amorphous quartz glass in a hollow mold which rotates about a vertical axis. The mold is made of a material which does not react with the quartz or quartz glass particles introduced therein when the quartz or quartz glass particles are heated. Filling can be continuous or in batches. As the mold rotates, a layer of quartz or quartz glass will form at the inner wall and on the bottom wall thereof. Heat is then applied from the interior of the mold to at least partially melt the introduced particles or powder of quartz or quartz glass. A thin partial layer will sinter. The remainder will be in a layer in granular form. After cooling, the quartz glass crucible can be removed from the hollow mold.
Manufacture of hollow bodies made of quartz by introducing comminuted starting material, such as quartz sand or granulated quartz crystals, in a rotatable hollow mold is described in German Patent DE-PS No. 543 957. The raw material is filled into a rotatable hollow mold, continuously or discontinuously, melted therein and sintered. Centrifugal force, upon rotation of the mold, presses the material against the inner wall of the mold. The hollow body is removed from the mold after cooling. The raw material is heated by a heat source positioned in the interior of the mold to heat the material to melting temperature or sintering temperature, respectively. In accordance with this process, hollow bodies are obtained which still contain air bubbles in their structure. To manufacture quartz glass bodies which are low in, or completely free from, air bubbles, it is necessary to sequentially introduce thin layers of raw material into the hollow crucible and to melt together the raw materials. Decrease in the air bubbles is obtained when air trapped in the granular raw material, during melting, at least partially escapes in the direction of the axis of rotation of the hollow mold.
It has been proposed to manufacture shaped hollow bodies of glass by introducing a glass tube into a hollow mold and, upon simultaneous rotation of the hollow mold and the tube, heating the tube and thus causing placement of the tube against the surface of the rotating hollow mold (see German Patent DE-PS No. 822 005). The bottom of the hollow mold may have suction openings applied thereto connected to a vacuum source. The bottom of the hollow mold which, of course, has only small or no centrifugal forces acting thereon, thus has applied a force against which the bottom of the glass is pressed, so that the glass will have the required shape as determined by the shape of the mold even in those regions where centrifugal forces are not active.
Crucibles, and other articles made of quartz glass or similar glasses, require structures which are essentially free from included air bubbles or other structural inperfections. Such articles are used in large quantities and, therefore, the manufacture thereof should be rapid and inexpensive, requiring as little applied energy for heating of the materials as possible.